Intravenous drug users are exposed to unique risks of infectious disease through their drug use and sexual behavior. The proposed research team will study the consequences if these risks through a transdisciplinary approach, combining epidemiology, biostatistics, immunology, and anthropology. The study will pursue scientific hypotheses suggested by prior investigation of relationships among key variables. The objective of the project is to build an understanding of these relationships across disciplinary lines in ways that will contribute to explanatory and predictive modeling of drug use, retroviral infection, immune status, and health status variables. A cohort of 300 IVDUs in Miami, Florida that has already been followed for three years will comprise the study population. The staff will continue to follow the cohort at six-month intervals, monitoring infection status, drug using and sexual risk behavior, immune status, and health status. Subsets of actively injecting subjects will contribute in-depth interviews on risk behavior during each wave of data collection. Results of these interviews will help to refine structured data gathering and analysis of health related hypotheses.